In certain types of nuclear reactors, such as light water reactors (LWR) including boiling water reactors (BWR) and pressurised water reactors (PWR) including those of Russian design VVER (Vodaa Vodiannee Energititscherski Reactor, in English Water Water Energy Reactor), the nuclear fuel is packed in nuclear fuel assemblies, each assembly comprising a bundle of nuclear fuel rods, each rod comprising a cylindrical cladding tube containing nuclear fuel and closing plugs welded to the ends of the cladding tube.
In operation, a heat transfer fluid (boiling water in BWR or pressurised water in PWR) circulates along the rods in order to extract the heat produced by the nuclear fuel. The heated fluid is then used in order to convert this heat into electrical energy.